As it is well known in the state of the art, running a fix pitch fan to high speed, it's necessary to design the twist of the rotor blades with high pitch angles. This implies the ejected air speeds to be high also in the low advancement speed (i.e. lending and take-off phases). But, high difference of speed amongst the inlet and the outlet of a fan mean great power to be supplied, low efficiency and high noise emissions. Hence from the 70's have been developed the turbo-fan with high by-pass ratio; these last allow accelerating less the higher capacity of air, achieving the same thrust but with higher efficiency, in respect with the jet.
It's also known that the best way to reduce the ejection air speeds in the low speed phases is to adapt the pitch of the rotor blades. In this manner, amongst the inlet and the outlet of a fan, it's possible to obtain reasonable difference of velocity in all the flight phases. Moreover, reducing the ejection air speed in the low flight velocity allows to abate the noise emission and to increase the static pressure downstream the fan. The increase of the static pressure, further associated to the higher rotor blades surface projected in the thrust sense, allows achievement of high thrust.
It's further known that the variable pitch fan can be used as a brake or as a thrust reverser, thus reducing the weight of the whole fan by eliminating the normal thrust reverser system.
It's for those reasons that the variable pitch fan, particularly for turbine engines, has been widely disclosed in the state of art. But no one arrangement has been yet developed and commercialised. The variable pitch rotor blades have been practically employed only in the open propeller, generally matched to turbo-prop.
The solutions proposed until today, which have some comparisons to this invention, are focused on solving the following matters: deal with the high dynamic turning moment due to the centrifugal forces (i.e. U.S. Pat. No. 3,870,434); reduce the loads due to actuator contained in the rotor (i.e. U.S. Pat. No. 3,922,852); realize a simple actuation system (i.e. U.S. Pat. No. 6,071,076); increase the overall efficiency modifying also the pitch of the stator blades (i.e. U.S. Pat. No. 5,911,679, U.S. Pat. No. 5,794,432 and U.S. Pat. No. 5,215,434); turn the rotor with a gearbox by way of an external engine (i.e. U.S. Pat. No. 3,146,755).
Currently, the turbine engines utilised in propulsion are predominantly of the Turbo-Engine type; as it is known, in this type of engines a turbine/compressor group rotates a power shaft to which a fixed pitch propeller located at the end of a divergent duct is connected; this duct called Air Intake, usually free of stator blades, has the scope to decelerate the air processed by the rotor in order to increase the efficiency.
These propulsion systems have the same limits of the fixed pitch propeller, which can be summarized as follows:                1. the efficiencies decrease very rapidly above defined speeds V of advancement;        2. the resultant of the applied forces coincides at the end of the blades, with consequent bending stresses which alter the system aerodynamics.        
In the Engines with ducted propellers, which have the scope to generate a thrust useful for the propulsion, none of the expedients which are proposed and justified in this analysis has been utilised.
In some jet engines, stator blade row (in some cases with movable twisted part) are located upstream of the rotor in the stages of the axial compressors, but to vary the performance modifying the pressure and to avoid the stall.
The variable pitch technique is instead widely utilised but only in he outside propellers for reasons that will be discussed hereinafter.